With the advance of an optical communication network having a high speed and a high capacity, needs for an optical signal processing device, a typical example of which is the wavelength division multiplexing (WDM) transmission signal processing, also has been increased. For example, a function has been required to switching a path of a multiplexed optical signal among nodes. By directly subjecting an optical signal to a path conversion without using an optical-electrical conversion, more optical signal processing devices having a higher speed have been made.
On the other hand, from a viewpoint of an optical signal processing device having a smaller size and integration, a planar lightwave circuit (PLC) has been researched and developed. In the PLC, a core made of silica glass is formed on a silicon substrate for example to integrate various functions on one chip to thereby realize an optical function device having a small loss and high reliability. Furthermore, a composite optical signal processing component (device) obtained by combining a plurality of PLC chips with other optical functional components also has been found.
For example, Patent Publication 1 discloses a signal processing device obtained by combining a PLC including arrayed-waveguide grating (AWG) for example with a spatial modulation device such as a liquid crystal device. More specifically, a wavelength blocker, a wavelength equalizer, a dispersion compensator or the like has been examined including a PLC with a collimate lens symmetrically arranged around a liquid crystal device at the center. In these optical signal processing devices, a plurality of optical signals having different wavelengths are subjected to an optical signal processing in an independent manner for each wavelength.
FIG. 9 is a conceptual diagram illustrating the configuration of an optical signal processing device. In this optical signal processing device, an optical signal is inputted and outputted through a spectroscopic element 100. The spectroscopic element 100 subjects a plurality of optical signals having different wavelengths to disperse at an angle depending on the wavelength thereof. The dispersed optical signal is output toward a focusing lens 2. The optical signal focused by the focusing lens 2 is focused, depending on the angle θ, to the respective focal points at predetermined positions of a signal processing device 4 having functions of intensity modulation, phase modulation, or steering. Specifically, it is noted that depending on the wavelength of the input optical signal, the optical signal is focused at different positions of the signal processing device. The signal processing device 4 is, for example, a liquid crystal device consisting of a plurality of element devices (pixels). By the control of the transmittance of the respective element devices or the like, the optical signals of the respective wavelengths are subjected to intensity modulation or the like to thereby realize a predetermined signal processing function. The optical signal subjected to the signal processing is reflected by a mirror 5 to have an inverted travelling direction. The optical signal further passes through a focusing lens 3 and is wavelength-multiplexed in the spectroscopic element 100. As is generally known, the spectroscopic element 100 also can subject the optical signal to the wavelength-multiplexing depending on the travelling direction. The wavelength-multiplexed optical signals having the respective wavelengths are outputted to the outside of the optical signal processing device as output light.
In FIG. 9, the spectroscopic element 100 is conceptually shown and may be the one that can perform wavelength-demultiplexing and wavelength-multiplexing depending on the wavelength of the optical signal. For example, the spectroscopic element includes grating, prism, AWG or the like. The signal processing device may be the one that can modulate the intensity or the phase of the optical signal or the intensity and the phase or that can steer the traveling direction of the optical signal. For example, the signal processing device includes, for example, a liquid crystal device, MicroElectro Mechanical Systems (MEMS) mirror, and nonlinear optical crystals.
The optical signal processing device shown in FIG. 9 is configured so that a mirror is used to reflect the optical signal so that one spectroscopic element performs both of the wavelength-demultiplexing and wavelength-multiplexing of the optical signal. This configuration is generally called the reflective-type one. The device for performing an optical signal processing such as wavelength block is not limited to this configuration. For example, another configuration is also possible where the mirror of FIG. 9 is not used and the signal processing device is positioned at a position of a symmetrical axis and the opposite side of the input system on the extended line of an input light path axis has an output system consisting of another lens and a spectroscopic element. This configuration is a configuration in which optical signal is dispersed and wavelength-multiplexing via the independent input system and output system, respectively and is called as the transmission-type one. Furthermore, by changing the angle of the mirror in the device configuration of FIG. 9, another configuration is also possible where an output system consisting of another lens and a spectroscopic element positioned at an arbitrary position is used to subject an optical signal to wavelength-multiplexing. For example, a configuration is also possible where the reflecting surface of the mirror is inclined by 45 degrees to an incident light path of the optical signal to provide a lens and a spectroscopic element placed in the vertical direction to the incident light path to thereby configure the output system. When the signal processing device has a steering function, the configuration also can include a plurality of output systems.
In FIG. 9, the spectroscopic element 100 is separated from the focusing lens 2 by a front focal length (FFL). The signal processing device 4 is separated from the focusing lens 2 by a back focal length (BFL). The position of the focal point of the light focused by the focusing lens 2 must be on the surface of the mirror 5 in all wavelengths used. If the position of the focal point of the light is dislocated from the mirror surface, a coupling loss between input and output lights is caused. Furthermore, a focused optical signal has an increased beam spot diameter. This consequently causes a disadvantage where the signal processing in the optical signal processing device has a declined wavelength resolution.
Furthermore, the signal processing device 4 must include a spatially-periodic structure in order to selectively perform modulation depending on each wavelength of an optical signal. For example, when the signal processing device 4 is a liquid crystal device, the structure of the element device of the liquid crystal device must be designed depending on the optical characteristics of the spectroscopic element and the focusing lens.
More specifically, it is known that the wavelength dependency of the focusing position on the signal processing device is determined by multiplying the angular dispersion value of the spectroscopic element with the focal length of the focusing lens. The wavelength dependency of the focusing position is also called a linear dispersion value of an optical system of a spectrometer. The linear dispersion value of the optical system determined by the spectroscopic element and the focusing lens must be close sufficiently to a linear dispersion value used for the design of the structure of the signal processing device. If these linear dispersion values are different from each other, the position of a focal point of an actual optical signal does not correspond to the position of each element device of the signal processing device (e.g., pixel of a liquid crystal shutter device). This inconsistency causes a wavelength error in a processed optical signal.    Patent Publication 1: Japanese Laid-Open Patent Application Publication No. 2002-250828 (page 16, page 19, FIG. 20, FIG. 27, FIG. 29D or the like)